Oxide superconductors of the rare earth-barium-copper-oxide family (YBCO), bismuth(lead)-strontium-calcium-copper-oxide family ((Bi,Pb)SCCO) and thallium-barium-calcium-copper-oxide family (TBCCO) form plate-like and highly anisotropic superconducting oxide grains. Because of their plate-like morphology, the oxide grains can be oriented along the direction of an applied strain. Mechanical deformation has been used to induce grain alignment of the oxide superconductor c-axis perpendicular to the plane or direction of elongation. The degree of alignment of the oxide superconductor grains is a major factor in the high critical current densities (J.sub.c) obtained in articles prepared from these materials.
Known processing methods for obtaining textured oxide superconductor composite articles include an iterative process of alternating anneal and deformation steps. The anneal is used to promote reaction-induced texture of the oxide superconductor in which the anisotropic growth of the superconducting grains is enhanced. Each deformation provides an incremental improvement in the orientation of the oxide grains. Heat treatment intermediate with or subsequent to deformation is also required to form the correct oxide superconductor phase, promote good grain interconnectivity and achieve proper oxygenation.
Processing long lengths of oxide superconductor is particularly difficult because deformation leads to microcracking and other defects which may not be healed in the subsequent heat treatment. Cracks that occur perpendicular to the direction of current flow limit the performance of the superconductor. In order to optimize the current carrying capability of the oxide superconductor, it is necessary to heal any microcracks that are formed during processing of the oxide superconductor or superconducting composite.
Liquid phases in co-existence with solid oxide phases have been used in processing of oxide superconductors. One type of partial melting, known as peritectic decomposition, takes advantage of liquid phases which form at peritectic points of the phase diagram containing the oxide superconductor. During peritectic decomposition, the oxide superconductor remains a solid until the peritectic temperature is reached, at which point the oxide superconductor decomposes into a liquid phase and a new solid phase. The peritectic decompositions of Bi.sub.2 Sr.sub.2 CaCu.sub.2 O.sub.8+x, (BSCCO-2212, where 0.ltoreq.x1.5), into an alkaline earth oxide and a liquid phase and of YBa.sub.2 Cu.sub.3 O.sub.7-.delta. (YBCO-123, where 0.ltoreq..delta.1.0) into Y.sub.2 BaCuO.sub.5 and a liquid phase are well known.
Peritectic decomposition of an oxide superconductor and the reformation of the oxide superconductor from the liquid+solid phase is the basis for melt textured growth of YBCO-123 and BSCCO-2212. For example, Kase et al. in IEEE Trans. Mag. 27(2), 1254 (Mar. 1991) report obtaining highly textured BSCCO-2212 by slowly cooling through the peritectic point. This process necessarily involves total decomposition of the desired 2212 phase into an alkaline earth oxide and a liquid phase.
It is also recognized that oxide superconductors, in particular, can coexist with a liquid phase under suitable processing conditions. This may occur by solid solution melting, eutectic melting or by formation of nonequilibrium liquids. Solid solution melting may arise in a phase system, in which the oxide superconductor is a solid solution. As the temperature (or some other controlling parameter) of the system increases (or decreases), the oxide superconductor phase changes from a solid oxide phase to a liquid plus oxide superconductor partial melt (this happens at the solidus). A further increase in temperature (or some other controlling parameter) affords the complete melting of the oxide superconductor (this happens at the liquidus).
A phase diagram containing a eutectic point may provide an oxide superconductor partial melt, known as eutectic melting, when the overall composition is chosen so as to be slightly off stoichiometry. As the temperature (or some other controlling parameter) of the system increases (or decreases), the mixed phase of oxide superconductor-plus-nonsuperconducting oxide (solid.sub.1 +solid.sub.2) changes to a liquid-plus-oxide superconductor partial melt (solid.sub.1 +liquid).
Non-equilibrium liquids may also promote partial melting in oxide superconductor systems. A non-equilibrium liquid is established, usually through rapid heating of a mixture of oxides to a temperature above their eutectic melting point. As the oxides form the desired oxide superconductor, the solid and liquid phases can co-exist, if only temporarily.
Partial melting of (Bi,Pb).sub.2 Sr.sub.2 Ca.sub.2 Cu.sub.3 O.sub.10+x ((Bi,Pb)SCCO-2223, where 0.ltoreq.x.ltoreq.1.5) and (Bi).sub.2 Sr.sub.2 Ca.sub.1 Cu.sub.2 O.sub.10+x ((Bi)SCCO-2223, where 0.ltoreq.x.ltoreq.1.5) at temperatures above 870.degree. C. in air has been reported; see, for example, Kobayashi et al. Jap. J. Appl. Phys. 28, L722-L744 (1989), Hatano et al. Ibid. 27(11), L2055 (Nov. 1988), Luo et al. Appl. Super. 1, 101-107, (1993), Aota et al. Jap. J. Appl. Phys. 28, L2196-L2199 (1989) and Luo et al. J. Appl. Phys. 72, 2385-2389 (1992). The exact mechanism of partial melting of BSCCO-2223 has not been definitively established.
Guo et al. in Appl. Supercond. 1(1/2), 25 (Jan. 1993) have described a phase formation-decomposition-reformation (PFDR) process, in which a pressed sample of (Bi,Pb)SCCO-2223 is heated above its liquidus to decompose the 2223 phase, followed by a heat treatment at a temperature below the solidus. The sample was subsequently pressed again and reannealed. The final anneal of the PFDR process includes a standard single step heat treatment in which there is no melting.
Partial melting in the processing of oxide superconductors has been used either to increase the yield of the BSCCO-2223 phase or to improve the contiguity and texturing of the oxide superconductor grains. The issue of healing defects, such as microcracks, which develop during processing of the oxide superconductor, has not been addressed. Further, the prior art has not addressed the possibility of using a two-step process where the oxide superconductor is stable in both steps for the healing of cracks and defects.
It is the object of the present invention to provide a method for improving superconducting performance of oxide superconductors and superconducting composites by healing cracks and defects formed during processing of oxide superconductors and superconducting composites.
It is a further object of the invention to prepare oxide superconducting articles having significantly less cracks and defects than conventionally-processed articles.
A feature of the invention is a final two-step heat treatment which introduces a small amount of a liquid phase co-existing with the oxide superconductor phase, and then transforms the liquid back into the oxide superconductor phase with no deformation occurring during or after the final heat treatment.
An advantage of the invention is the production of highly defect-free oxide superconductor and superconducting composites which exhibit superior critical current densities.